GREENS EXPRESSION FOR SATURATION CURRENT VS. BANDGAP: APPLICATIONS IN SI AND CIGS SOLAR CELLS

摘要

We present two applications of the Green's expression for saturation current vs. bandgap (E_g). The first one regards the evaluation of E_g in solar cells. The approach is based on the determination of the saturation current in the quasi-neutral regions (QNR) to be inserted into the Green's expression to find E_g. The saturation current in the QNR is obtained by fitting the Ⅰ-Ⅴ curves of the device under exam with a reported method using the double-diode model. A validation method is also given. The application is tested on the cells of Si and CIGS modules, whose performance parameters are measured. Due to the lack of information about the material parameters of the module cells no certain conclusions about the accuracy of the found E_g-values can be drawn. In Si, however, all E_g-values are lower than the intrinsic value, which is expected due to doping. The second application aims at the characterization of CIGS solar cells with respect to the kind of prevailing recombination in QNR by means of fits to performance data vs. E_g. To test the application, we have collected published performance-data vs. E_g for various sets of CIGS cells. Successful results are obtained. Using the Green's expression for saturation current vs. E_g to fit the performance data vs. E_g allows in fact ascertaining from the value to be assigned to a parameter what kind of recombination is overcoming in the QNR of the relevant cells.